Devices for releasing a controlled dose of medication may be actuated electronically. Such devices are generally referred to as inhalers, inhaling devices, breath inhaling devices, and breath-operated inhaling devices. Historically, the patient actuates inhalers mechanically as he or she inhales. Difficulty arises for many patients in coordinating breathing and actuating the medicine delivery. Newer devices in development can now deliver medicine automatically and more precisely by measuring the flow rate and triggering the actuator as the patient inhales. At a certain flow rate, the lung passages are detected as open and the output of a sensor may be fed back to a comparator or microprocessor to actuate medicine delivery. To trigger such a device, either a pressure sensor or a flow sensor generally senses flow. A pressure sensor may be ported to sense pressure drop created by flow through the mouthpiece assembly or through an orifice in the mouthpiece assembly. But with pressure sensing technology, larger diaphragms are typically needed to sense the very low pressure drop required with breath-operated devices. As a consequence, devices with the larger size diaphragm cost more.
Two types of flow sensors are regularly used to measure flow in medical equipment: a hot wire anemometer or its miniature form, the silicon-based thermal microsensor (also known as the micro-bridge airflow sensor or thermally based Micro Electronic Mechanical System (MEMS) device). A flow sensor is typically deployed by placing it directly in the flow stream; but flow sensor wire bonds or sensing elements may be damaged by debris, lint, or from patient mishandling. Hot wire anemometer flow sensors have the disadvantage of requiring relatively high power. Although thermal microsensor type flow sensors operate with lower power, flow turbulence generated within the breath-operated device and extraneous flow in ambient air cause signal errors when placed directly in the flow.
U.S. Pat. No. 5,469,750, entitled “Method and Apparatus for Sensing Flow in Two Directions and Automatic Calibration Thereof”, is directed to a method of calibrating the output of a transducer in a flow path through which a medication is delivered from a hand-held metered dose inhaler. A transducer senses flow rates of human breath during inhalation and exhalation through a portion of a flow path in the inhaler. The flow rates have two non-linear flow characteristics in opposite directions. The flow path portion provides flow rate measurements representative of flow paths of the entire system. U.S. Pat. No. 5,622,162, entitled “Method and Apparatus for Releasing a Controlled Amount of Aerosol Medication Over a Selectable Time Interval”, is directed to another portable, but battery powered, hand-held system for releasing a controlled dose of aerosol medication for inhalation by a patient, including a durable body and a medication cassette inserted in the durable body. The cassette includes a housing for containing a canister of medication, bears an identification code, and permits the canister to be manually depressed to release a dose, e.g., a metered dose, when out of the durable body. The durable body includes an actuator mechanism for engaging an inserted cassette and its canister and an actuator release mechanism for controlling the actuator mechanism to depress the canister for a selected period of time in order to release the desired dose of medication and then the release the canister. The actuator mechanism includes a compression spring for depressing the canister and a torsion spring for reloading the compression spring. The torsion spring is reloaded by rotating the cassette from an open position for delivering aerosol to a closed position. The actuator-release mechanism includes a motor and trigger pin assembly that controls the release of the compression spring and the torsion spring, and, hence, the time that the canister is depressed. The motor operates in response to sensed flow satisfying a selected delivery threshold. The durable body includes a flow sensor having an asymmetrical orifice that is calibrated, independent of the cassette, to convert the sensed pressure due to flow into a flow rate. The orifice is separately calibrated for an inhalation flow rate range and an exhalation flow rate range over a selected number of known flow rates. The sensed pressure value is corrected for transducer offset drift and converted to a flow rate using the calibration data and piecewise linear interpolation.
Generally, prior art inhalation device sensors sense flow through a general flow path formed in the inhalation device by using a pressure sensor to measure pressure drop or by mounting a flow sensor directly in the flow path. The present inventors have recognized that a more sensitive technique is needed. The present inventors have invented and herein describe an indirect flow sensing technique that uses flow outside of the direct flow path associated with the sensing apparatus. Airflow can instead be driven through a low resistance channel across the airflow sensor using the present invention. Such a technique is more sensitive at very low flows than the pressure sensor technology described by the prior art and more durable than having an airflow sensor mounted directly in the flow channel.
The indirect flow sensing method can also be incorporated in an inhalation device that allows for the closure of the device's airflow ports by allowing for the rotation of the mouthpiece assembly from open to closed positions relative to the inhaling device's main body and handle assembly.
Accordingly, the present invention is described and presented to address the shortcomings currently found with the prior airflow sensing art.